In all forms of signal transmission, a signal is transmitted over a channel or transmission line. The channel or transmission line includes a physical medium through which the signal travels, such as a wire, optical fiber, or the atmosphere. Such data transmission systems are referred to below interchangeably as "channels" or "transmission lines," both terms being taken to mean any physical system through which a signal can travel. Another aspect of the channel is the readiness of signal handling elements that make up the channel to receive or send signal components. For signals to travel over the channel smoothly, there must be no physical barriers that disrupt the physical integrity of the channel, such as breaks in wires, or interfering structures that obstruct signals such as radio, or optical Further, receiving signal handling elements must be ready to accept a signal that has been transmitted by transmitting elements in the line, and signal transmitting elements must be ready to send a signal when other elements need to receive one. By ready, it is meant properly warmed up, booted, etc., and not occupied by other tasks.
Often the data rate state of different transmitter/receiver ("transceiver") components in a transmission line are mismatched. This causes problems. Specific instances of the problem abound. For instance, a radio frequency transmission is made from a transmitter to a receiver. In a digital system, as shown schematically in FIG. 1, the transmitting user pushes a button 106 to initiate the transmission, then initiates the generation of a signal or speaks into a microphone 100. The microphone is connected to an analog to digital converter 102, which is connected to a modulator 104. The converter converts the analog electric signal to a digital signal representing the sound made by the user, for instance, speech.
The modulator 104 manages the stream of digital signals so that it is in a form that can be transmitted over the channel 108. The digital, electric signal is transduced into an electromagnetic radiation, radio frequency signal, and broadcast through the medium of the atmosphere. At the receiver 110, the radiation is picked up by a receiving apparatus, such as an antenna, passed through demodulator 112, transformed into an analog electric signal by D/A converter 114 and transformed into whatever form of signal is desired, for instance, an analog acoustic signal such as voice, generated by a loud speaker 116.
The problem arises because after the transmitting operator pushes the start button 106, it takes a finite period of time before the radio communication channel 108 is "open." By open, it is meant before the components that make up the receiver 110 and transmitter 120 are mutually in condition to receive a signal that has been transmitted. For a typical radio link, the time to open the channel may be on the order of between one millisecond and two seconds. Thus, if the transmitting operator begins talking immediately or any time before the channel 108 is open the first moments of the signal will not be received by the receiver 110. Part of the signal will be lost.
Not only is this phenomenon annoying, but it also can present circumstances that defeat or hamper the purposes of the communication. For instance, in a military or police situation, if the message to be transmitted is a spoken, curt "Don't Shoot!" and the channel does not open until after the transmission of the word "Don't," the message received is "Shoot!" which is wrong, drastically so. Many other scenarios exist where the initial portion of the signal to be transmitted is crucial to the message intended.
One way that the radio transmission problem has been addressed is to run the entire signal through a buffer 122, which delays signal transmission for a brief period of time, until the channel has opened. One drawback to this approach is that the entire signal is then delayed. If the signal is being used to synchronize or otherwise coordinate activities happening at different physical locations, the ongoing delay presents a problem. Further, if the channel closes unexpectedly while the transmitter is transmitting, and then opens again at a later time, information will be lost due to the subsequent channel closing.
Another instance where channel integrity problems arise is in the context of several different signal transceiver components in a transmission line that transmit or receive a signal. Each transceiver component may be driven by its own clock. The clocks may be running at nominally different speeds (e.g. 8 kHz and 16 kHz). Further, the nominal speeds may not be absolutely fixed, with each clock tending to drift with respect to a significantly more stable standard, and thus, with respect to the nominal and actual speed(s) of the other clock(s). Therefore, it is difficult to coordinate the flow of the signal through the signal components.
For instance, as shown schematically in FIG. 2A, in a voice transmission system 200, the components of one portion of the system may include an analog to digital converter 202 running under the influence of a first clock 204, which digitizes an analog signal AS (FIG. 2B) by sampling its amplitude at a specified frequency F.sub.1. The sampled signal is presented to modulator 208, which is run by a different clock 210.
The A/D converter 202 digitizes the amplitude of analog signal AS every n cycles of clock 204. The modulator 208 takes a packet of a certain number of signal samples every w cycles of clock 210 and puts those packets in a form that can be used by the rest of the system, such as a radio link channel.
Ideally, the rate at which A/D converter 202 generates signal samples, and the rate at which modulator 208 accepts packets of signal samples, are correlated, so that the modulator 208 is ready to accept a packet of sampled signals when it is presented, and vice versa. If the clocks 204 and 210 were perfectly regular, this would be simple to accomplish. For instance, if A/D converter 202 runs at 16 kHz and samples once every 20 cycles or 1/800 second (200 samples every 1/4 second) and modulator 208 runs at 8 kHz and takes a packet of 200 samples every 2000 cycles (1/4 second), everything will work smoothly. (These sampling rates are artificial, designed for discussion purposes only.) However, the clocks are not perfect, each one drifting from its nominal value. Consequently, over time, the A/D converter 202 and modulator 208 can get out of synchronization, and samples will be presented to modulator 208 before it is ready to handle it or it will call for samples before they are ready.
For instance, if the clock 204 beats slightly faster than the clock 210, such that every 1/4 second A/D converter samples 220 times, after 1/4 second, 220 samples will be available, which is more than the 200 that modulator 208 can handle. Samples will be dropped. A similar problem arises if clock 204 beats too slowly. If samples are not available when called for, the system will send spurious signals.
Known systems have addressed this problem simply by periodically discarding a portion of the signal when a backlog arises or sending default signals if a deficit arises. This is undesirable, because the signal quality is compromised. It is not possible to simply use a large buffer, because, if there is a mismatch that persists for a long enough time, any buffer will overflow, no matter how large it is. Similarly, to overcome a deficit, it would not be possible to begin with a stockpile of buffered samples, because over a long enough time, the stockpile would run empty.
Thus, it is an object of the invention to provide an apparatus that maintains the integrity of a signal, despite a mismatch in the readiness of transceiver components that make up the channel, or transmission line over which the signal will pass. It is an object of the invention to provide an apparatus and a method for transmitting speech over a channel, such that no information content of the speech is lost, despite channel interruptions and such that a permanent buffered delay is not established. It is another object of the invention to provide such an apparatus that does not distort important aspects of a speech signal, such as pitch and frequency. Yet another object of the invention is to transmit data in a system having more than one clock and to avoid the problems associated with the drift in the clocks.